Plateau of practice effects and noise with repeat SDMT testing in multiple sclerosis

Area of Science:

• Neuroscience
• Clinical Psychology
• Rehabilitation Medicine

Background:

• The Symbol Digit Modalities Test (SDMT) is a primary cognitive assessment for multiple sclerosis (MS).
• Repeated SDMT administration is subject to practice effects and test-retest variability, complicating longitudinal tracking of cognitive function in people with MS (PwMS).

Purpose of the Study:

• To quantify the build-up and plateau of SDMT practice effects with high-frequency testing.
• To establish reliable cutoffs for detecting longitudinal cognitive change in PwMS.
• To determine if increased testing frequency enhances the detection of cognitive decline.

Main Methods:

• PwMS underwent monthly SDMT testing over three years.
• Plateau regression analysis identified practice effect inflection points.
• Reliable change was assessed, and findings were compared to a low-density testing group.

Main Results:

• Practice effects on the SDMT plateaued after 18 administrations (p < 0.001).
• A minimum 7-point change in SDMT score was required for 90% confidence in detecting cognitive decline.
• Increased testing frequency did not significantly improve the detection of cognitive decline (p = 0.256).

Conclusions:

• Practice effects in the SDMT are substantial, requiring up to 18 administrations to stabilize.
• Test-retest variability observed is consistent with existing literature.
• Findings necessitate the use of alternate SDMT forms and reliable change methodologies for accurate cognitive monitoring in PwMS.